Combustors are commonly used in industrial and power generation operations to ignite fuel to produce combustion gases having a high temperature and pressure. Various competing considerations influence the design and operation of combustors. For example, higher combustion gas temperatures generally improve the thermodynamic efficiency of the combustor. However, higher combustion gas temperatures also promote flashback or flame holding conditions in which the combustion flame migrates towards the fuel being supplied by nozzles, possibly causing severe damage to the nozzles in a relatively short amount of time. In addition, higher combustion gas temperatures generally increase the disassociation rate of diatomic nitrogen, increasing the production of nitrogen oxides (NOx). Conversely, lower combustion gas temperatures associated with reduced fuel flow and/or part load operation (turndown) generally reduce the chemical reaction rates of the combustion gases, increasing the production of carbon monoxide and unburned hydrocarbons.
In a particular combustor design, an end cap may extend radially across a portion of the combustor, and a plurality of tubes may be radially arranged in the end cap to provide fluid communication through the end cap and into a combustion chamber. A working fluid and fuel are supplied through the tubes to enhance mixing between the working fluid and fuel before reaching the combustion chamber. The enhanced mixing allows leaner combustion at higher operating temperatures while protecting against flashback or flame holding and controlling undesirable emissions. However, some fuels supplied to the tubes produce vibrations in the combustor that may lead to harmful combustion dynamics. The combustion dynamics may reduce the useful life of one or more combustor components. Alternately, or in addition, the combustion dynamics may produce pressure pulses inside the tubes and/or combustion chamber that affect the stability of the combustion flame, reduce the design margins for flashback or flame holding, and/or increase undesirable emissions. In addition to combustion dynamics, other common sources of vibration in the combustor may be caused by rotor vibrations, rotating blade frequencies, and flow induced vibrations associated with vortex shedding.
Various efforts have been made to reduce the vibrations produced by fluid flow through the end cap. For example, various structures and methods have been developed to prevent or avoid harmonic frequencies from being created in the combustor. Alternately or in addition, the volume or geometry of the combustor may be adjusted to change the natural or resonant frequency of components in the combustor; however, the change in volume or geometry may adversely affect the mixing between the fuel and working fluid. As an alternative or additional approach, increasing the natural or resonant frequency of the end cap in the combustor may be useful to avoiding harmonic frequencies in the combustor and the associated undesirable combustor dynamics.